Nitrogen foam fracturing is a novel technology developed on the basis of the hydraulic fracturing technology and is in particular suitable for improving the low-pressure, low-permeable and water-sensitive formulation during the oil-gas field exploitation and the coal bed methane exploitation. Such technology is also widely applied in the technical field of nitrogen flooding exploitation for coal gas methane. In the existing nitrogen foam fracturing technology, the overall testing technology for the nitrogen foam fracturing equipment is not yet mature and the experiment apparatus is not yet consummate. In particular, the liquid nitrogen pump, as an important equipment among others, requires a set of integrated apparatus for the overall test and experiment of liquid nitrogen pump equipments.
The liquid nitrogen pump equipment is a specialized equipment that discharges liquid nitrogen through a liquid nitrogen pump under pressure and then heats the high pressure liquid nitrogen by a specialized heating apparatus so as to convert it into gaseous nitrogen at normal temperature. The typical characteristic of the liquid nitrogen pump equipment lies in the fact that the input medium is low temperature liquid medium but the output is high pressure gas with large displacement. The liquid pump equipment is an important equipment applied in the oil-gas field and coal gas methane foam fracturing, and thus requires periodical checkout so as to ensure the security of the field operation. At present, mainly utilized is the loading experiment apparatus for liquid nitrogen pump equipments in the overall test and experiment of the liquid nitrogen pump equipment. The loading experiment apparatus for liquid nitrogen pump equipments mainly works in testing and experimenting on both the displacement and pressure of the discharged gas of the liquid nitrogen pump equipment so as to check out the performance of the liquid nitrogen pump equipment.
The output gas displacement of the liquid nitrogen equipment in practical application depends on the power transmitting system and the flow distributing system of the equipment, whereas the pressure of the output gas displacement depends on the factors comprising fracture pressure of the formation, formation permeability, and pipe resistance, etc., during the foam fracturing operation completed thereby. In consideration that the fracture pressure of the formation, the formation permeability, and the pipe resistance are relatively constant during the experiment, it can not be readily realized to adjust the discharge pressure of the gas discharged from the liquid nitrogen pump equipment, and thus such apparatus is not appropriate to be used as experiment technology for the liquid nitrogen pump equipment. Nowadays, the domestic test and experiment apparatus is only able to satisfy the requirement of overall test and experiment of the liquid nitrogen pump equipment with relatively low pressure and low displacement in China. However, the liquid nitrogen pump equipment used for nitrogen foam fracturing has the characteristics as follows: large installed power, which reaches 1500 kW; large displacement, which reaches 600 Sm3/min (Sm3 means normal cubic meter and is specialized as an unit of gas volume. Sm3 refers to volume of gas under standard temperature and pressure, wherein S is the initial of English word “standard”); high discharge pressure, which reaches 140 MPa; the test and experiment of such equipment substantially differ from those of the liquid nitrogen pump equipment with low pressure and low displacement in terms of security risk. Accordingly, the requirements to the overall test and experiment of such equipment will significantly differ from those to normal liquid nitrogen pump equipment with low pressure and low displacement, mainly in terms of the pressure loading technology and also the data collecting and analyzing technologies. As for the liquid nitrogen pump with low displacement, the loading of discharge pressure may be carried out using a loading method by directly regulating valves under complete safe security; the data collecting and analyzing may also be directly obtained during the test and experiment of the liquid nitrogen pump equipment with low pressure and low displacement. However, as for the test and experiment of the liquid nitrogen pump equipment with high pressure and large displacement, a huge pressure regulating valve will be needed so as to realize the adjustment of discharge pressure if using the method of direct pressurizing mentioned above, which is technically very difficult to implement. On the other side, direct adjustment of gas with high pressure and large displacement renders high security risk for adjusting and monitoring, and thus results in high security risk during the test and experiment. Therefore, at present, there is urgent requirement for a test and experiment technology that is appropriate for foam fracturing liquid nitrogen pump equipment with high pressure and large displacement.